This application claims priority to an application entitled xe2x80x9cCircuit and Method for Compensating for Non-linear Distortionxe2x80x9d filed in the Japanese Patent Office on Mar. 22, 2001 and assigned Serial No. 2001-83715, and an application entitled xe2x80x9cCircuit and Method for Compensating for Non-linear Distortionxe2x80x9d filed in the Japanese Patent Office on Mar. 22, 2001 and assigned Serial No. 2001-83716, the contents of both of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to a quadrature modulation circuit used in a radio transmitter, and in particular, to an apparatus and method for compensating for non-linear distortion generated during high-power amplification after quadrature modulation of a baseband signal.
2. Description of the Related Art
A conventional quadrature (or orthogonal) modulation circuit quadrature-modulates a baseband signal and then high-power amplifies the modulated signal. The high-power amplified modulated signal is subject to non-linear amplification in order to improve power efficiency. This is because an amplification region of an amplifier is divided into a linear region and a non-linear region, and the high-power amplification is performed in the non-linear region. When amplified in the non-linear region, the amplified modulated signal suffers non-linear distortion. Thus, in order to linearize an input/output characteristic, it is necessary to compensate for distortion of the nonlinearly distorted signal. A typical, conventional non-linear distortion compensation circuit includes a predistortion-type non-linear distortion compensation circuit shown in FIG. 6.
A predistortion-type non-linear distortion compensation circuit will be described with reference to FIG. 6. Referring to FIG. 6, complex baseband signals I and Q are applied to a first D/A. (Digital-to-Analog) converter 2 and a second D/A converter 3 through a distortion compensation operator 1. The first and second D/A converters 2 and 3 convert received digital signals to analog signals, and provide the converted analog signals to a quadrature modulator 4. The quadrature modulator 4 quadrature-modulates received baseband signals I and Q, and provides the quadrature-modulated signals to a high-power amplifier (HPA) 5. The high-power amplifier 5 then high-power amplifies the quadrature-modulated analog signals.
A compensation data table 7 stores compensation data in the form of a table. The compensation data stored in the compensation data table 7 is determined by previously measuring a non-linear characteristic of the high-power amplifier 5 during amplification. A power calculator 6 calculates power of the baseband signals I and Q, and provides the calculated power information to the compensation data table 7. The compensation data table 7 reads compensation data corresponding to the calculated power by consulting the table according to the power of the baseband signals I and Q, and then provides the read compensation data to the distortion compensation operator 1.
In this way, the distortion compensation operator 1 applies an inverse distortion component for canceling the non-linear distortion generated in the high-power amplifier 5 to the received baseband signals I and Q before quadrature modulation. The signals including the inverse distortion component for removing the non-linear distortion are provided to the first and second D/A converters 2 and 3. As a result, the non-linear distortion of the modulated signals amplified by the high-power amplifier 5 is reduced.
As stated above, the conventional predistortion-type non-linear distortion compensation circuit compensates for non-linear distortion through the use of the data table based on the power of the baseband signals, without considering a characteristic deviation of the high-power amplifier 5 and an effect of a variation of temperature has on the HPA 5. Therefore, overall performance of the circuit may be deteriorated due to the characteristic deviation of the high-power amplifier 5 and the temperature variation.
In an attempt to solve this problem, a directional combiner 8, as illustrated in FIG. 7, divides an output of the high-power amplifier 5 into two signals, and applies one of the divided signals to a quadrature demodulator 9. The quadrature demodulator 9 quadrature-demodulates the divided signal and feeds the demodulated divided signal back to a compensation data operator 10. The compensation data operator 10 multiplies a coefficient based on the feedback information by data read from an internal compensation data table (though not shown, it is equal to the compensation data table 7 of FIG. 6). As a result, the compensation data operator 10 provides the distortion compensation operator 1 with compensated data having greater accuracy regardless of the characteristic deviation of the high-power amplifier 5 and the temperature variation.
However, since the elements 8-10 of FIG. 7 generate pseudo non-linear distortion themselves, it is not possible to completely resolve the problem. In addition, all these elements perform a complicated digital operation, resulting in an increase in the circuit size and cost. Further, the increase in the circuit size increases power consumption, causing a reduction in a batter-run time of a mobile communication terminal that uses a battery as a power source.
In a further attempt to solve this problem, the applicant has proposed a nonlinear distortion compensation circuit of FIG. 5, disclosed in Japanese patent application No. 2000-233631, the contents of which are hereby incorporated by reference. The nonlinear distortion compensation circuit includes directional combiners/dividers 19 and 21, a delay circuit/phase shifter 20, an attenuator 13, a subtracter 14, a quadrature modulator 11, a quadrature demodulator 15, a phase adjuster 22, amplitude adjusters 23 and 24, and subtracters 16 and 17.
The non-linear distortion compensation circuit interposes the directional combiner/divider 19 between the quadrature modulator 11 and a high-power amplifier 12. The directional combiner/divider 19 divides a modulated signal provided from the quadrature modulator 11 into two signals, and provides one of the divided modulated signals to the delay circuit/phase shifter 20 and provides the other divided modulated signal to the high-power amplifier 12. The delay circuit/phase shifter 20 then shifts the phase of the received signal to match it to the phase of an output signal of the attenuator 13, and then provides the phase-shifted signal to the subtracter 14. Also, the directional combiner/divider 21 divides the output of the high-power amplifier 12 into two signals, and provides one of the divides signals to the attenuator 13. The subtracter 14 calculates a difference between the signal from the delay circuit/phase shifter 20 and the signal from the attenuator 13, and provides the difference to the phase adjuster 22. The non-linear distortion component calculated by the subtracter 14 is phase-adjusted through the phase adjuster 22, and then provided to the quadrature demodulator 15. Baseband non-linear distortion components output from the quadrature demodulator 15 are amplitude-adjusted to a proper level through the amplitude adjusters 23 and 24, and then provided to the subtracters 16 and 17. A non-linear distortion extractor 1A for extracting the non-linear distortion component from the non-linearly high-power amplified modulated signal is comprised of the directional combiners/dividers 19 and 21, the delay circuit/phase shifter 20, the attenuator 13 and the subtracter 14.
However, this conventional distortion compensation circuit controls the delay circuit/phase delay 20 and the attenuator 13 on the basis of the point in time when the high-power amplifier 12 has its maximum output power where the non-linear distortion is most significant. Therefore, a reference distortion compensation control point is fixed to the point where the high-power amplifier 12 has its maximum output power where the non-linear distortion is most significant. Thus, the subtracter 14 outputs the nonlinear distortion component with highest precision at the fixed reference control point where a distortion-free signal is minimized. Though an adjacent channel power ratio (ACPR) is not considerably deteriorated even at the non-maximum output power as compared with at the maximum output power, the control point may be slightly deviated, resulting in deterioration of the system performance. In addition, a subtle change in the non-linear characteristic of the high-power amplifier 12 due to a variation in the time and the ambient temperature may deviate an optimal control point.
Further, as illustrated in FIG. 8, an input power-to-ACPR characteristic of the high-power amplifier 12 in the non-linear distortion compensation circuit is greatly deteriorated when the distortion compensation is continuously performed as compared with when the distortion compensation is suspended at a specific input power level. The reasons are as follows. The non-linear distortion compensation circuit controls the delay circuit/phase shifter 20 and the attenuator 13 at the point when the high-power amplifier 12 has its maximum output power where the non-linear distortion is most considerable, and fixedly sets a point a showing the non-linear distortion component with the highest accuracy, where the distortion-free signal is minimized, as the control point of the delay circuit/phase shifter 20 and the attenuator 13. Accordingly, a change in the output power of the high-power amplifier 12 may cause deviation of the control point, so that the subtracter 14 cannot completely remove the distortion-free signal. That is, the signal at the output node a of the subtracter 14 includes the distortion-free signal, which is an output signal of the transmitter, as well as the non-linear distortion component.
As a result, when the input power level of the high-power amplifier 12 is lower than a predetermined level, even the desired distortion-free signal component is fed back to the quadrature modulator 11. The feedback of the desired distortion-free signal component causes a decrease in a level of the desired distortion-free signal component, resulting in a reduction in the ACPR.
Therefore, it is an object of the present invention to provide a non-linear distortion compensation circuit and method for extracting non-linear distortion with high accuracy regardless of a variation of the time and the ambient temperature in a high-power amplifier.
It is another object of the present invention to provide a non-linear distortion compensation circuit and method for providing an excellent ACPR characteristic over the whole input power level of a high-power amplifier.
In accordance with one embodiment of the present invention, there is provided a method for compensating for non-linear distortion generated during non-linear high-power amplification in a transmitter after quadrature modulation, the transmitter including a distortion extractor for extracting a non-linear distortion component from a non-linearly high-power amplified modulated signal, a quadrature modulator for quadrature modulating the non-linear distortion component into a baseband distortion component after phase adjustment, and a distortion overlapping section for overlapping the baseband signal with a phase-inverted distortion component of the baseband distortion component. The method comprises attenuating an output signal of the high-power amplifier by a gain of the high-power amplifier when the high-power amplifier generates an amplified signal, and extracting only a non-linear distortion component by subtracting a phase-adjusted input signal of the high-power amplifier from the attenuated signal, converting the non-linear distortion component into a DC component, and automatically controlling an attenuation of the output signal of the high-power amplifier so that the DC component becomes minimized.
In accordance with another embodiment of the present invention, there is provided a circuit for compensating for non-linear distortion generated during nonlinear high-power amplification in a transmitter after quadrature modulation, the transmitter including a distortion extractor for extracting a non-linear distortion component from a non-linearly high-power amplified modulated signal, a quadrature modulator for quadrature modulating the non-linear distortion component into a baseband distortion component after phase adjustment, and a distortion overlapping section for overlapping the baseband signal with a phase-inverted distortion component of the baseband distortion component. The circuit comprises a converter for converting the non-linear distortion component output from the distortion extractor into a DC component; and a controller for automatically controlling an attenuation of the output signal of the high-power amplifier so that the DC component output from the converter becomes minimized.
The distortion extractor comprises an attenuator for attenuating the output signal of the high-power amplifier by a gain of the high-power amplifier, when the high-power amplifier outputs an amplified signal; a phase adjuster for adjusting a phase of an input signal of the high-power amplifier; and a subtracter for subtracting an output signal of the phase adjuster from an output signal of the attenuator. The attenuator, under the control of the controller, automatically controls an attenuation of the output signal of the high-power amplifier such that the DC component output from the converter becomes minimized.
In accordance with a further embodiment of the present invention, there is provided a method for compensating for non-linear distortion generated during non-linear high-power amplification in a transmitter after quadrature modulation, the transmitter including a distortion extractor for extracting a non-linear distortion component from a non-linearly high-power amplified modulated signal, a quadrature modulator for quadrature modulating the non-linear distortion component into a baseband distortion component after phase adjustment, and a distortion overlapping section for overlapping the baseband signal with a phase-inverted distortion component of the baseband distortion component. The method comprises suspending an operation of overlapping the baseband signal with a phase-inversed distortion component of the baseband distortion component, when an input power level of the high-power amplifier drops below a power level at a point where an input power-to-ACPR characteristic of the high-power amplifier becomes worse during non-linear distortion compensation control than during suspension of the non-linear distortion compensation control.
In accordance with yet another embodiment of the present invention, there is provided a circuit for compensating for non-linear distortion generated during non-linear high-power amplification in a transmitter after quadrature modulation, the transmitter including a distortion extractor for extracting a non-linear distortion component from a non-linearly high-power amplified modulated signal, a quadrature modulator for quadrature modulating the non-linear distortion component into a baseband distortion component after phase adjustment, and a distortion overlapping section for overlapping the baseband signal with a phase-inverted distortion component of the baseband distortion component. The circuit comprises a switch for switching an input signal, included in a feedback loop formed between the distortion extractor and the distortion overlapping section; a power detector for detecting an input power level of the high-power amplifier; a controller for receiving the signal detected by the power detector and turning off the switch when the input power level of the high-power amplifier drops below a power level at a point where an input power-to-ACPR characteristic of the high-power amplifier becomes worse during non-linear distortion compensation control than during suspension of the non-linear distortion compensation control.
In accordance with still another embodiment of the present invention, there is provided a circuit for compensating for non-linear distortion generated during non-linear high-power amplification in a transmitter for non-linearly high-power amplifying a baseband signal by a high-power amplifier after quadrature modulation, the transmitter including a distortion extractor for extracting a non-linear distortion component from the non-linearly high-power amplified modulated signal, a quadrature modulator for quadrature modulating the non-linear distortion component into a baseband distortion component after phase adjustment, and a distortion overlapping section for overlapping the baseband signal with a phase-inverted distortion component of the baseband distortion component. The circuit comprises a power detector for detecting an input power level of the high-power amplifier; and a controller for receiving the signal detected by the power detector and disabling one of a plurality of function elements included in a feedback loop formed between the distortion extractor and the distortion overlapping section, when the input power level of the high-power amplifier drops below a power level at a point where an input power-to-ACPR characteristic of the high-power amplifier becomes worse during non-linear distortion compensation control than during suspension of the non-linear distortion compensation control.